1. Field of the Invention
The present invention relates to a method of measuring reduction of surface layer material and to an apparatus therefor. More specifically, the invention relates to a method of measuring reduction of surface layer material utilizing activation by neutrons and activation by charged particles and to an apparatus therefor.
2. Prior Art
Machinery and structures, made of various materials, in general, lose their surface layer materials with use because of action such as wear, corrosion, or erosion. A method of measuring such surface layer material reduction by utilizing activation by neutrons and activation by charged particles has heretofore been known. This method comprises essentially the following four steps.
First step (activation)--portions of two objects, which are a component to be actually measured and a reference calibration part, and which are made of the same material, are activated.
Second step (calibration)--a relationship is found for the reference calibration part between the radioactivity and the amount of reduction.
Third step (measurement)--radioactivity is measured for the component while the surface layer is being reduced gradually.
Fourth step (analysis)--the amount of reduction is found from the radioactivity measured in the third step and from the reference calibration obtained in the second step.
However, this method involves problems with regard to the following two points.
(1) Positional relationship between a detector and an object to be measured.
With reference to the above-mentioned third step (measurement), it was not so far possible to change the positional relationship between the detector and the object to be measured during the measurement. That is, the count rate and the amount of reduction could not be related to each other unless the intensity of gamma-rays was measured while maintaining the same positional relationship between the object to be measured and the detector. The intensity of gamma-rays at this positional relationship, was 100% at zero reduction on the calibration curve. This is because a change in the positional relationship between the detector and the object to be measured during the measurement results in a change in the counting efficiency of the detector. The counting efficiency of the detector is represented by the ratio of how much gamma-rays the detector detects out of the total gamma-rays emitted by the object that is to be measured. This varies depending upon how much gamma-rays reach the detector out of the total gamma-rays emitted by the object to be measured, but neglecting the specific response of the detector to the gamma-rays. That is, the counting efficiency for the gamma-rays decreases with the increase in the amount of obstacles and/or the distance that exist between the detector and the object to be detected, and the counting efficiency increases with the decrease in the amount of obstacles and/or the distance. As a result according to the conventional method, the count rate markedly decreases and error increases when the reduction of surface layer material proceeds and radioactivity remaining in the surface layer greatly decreases.
(2) Shape of beam
Charged particles travel in a straight path in a vacuum. Microscopically, however, their paths in a substance are not straight, the paths are bent due to interaction by the atomic nuclei, and the particles travel while losing their energy. When an irradiation field is great to some extent, a multi-bending path of the incident beam in the target substance does not matter much because cross sections of the activated region through the whole depth appear identical with a cross section of the incident beam. In practice, however, the edges swell outwardly beyond the circumference of the incident beam due to multi-bending of the path, but there is almost no effect on the calibration relationship since the swollen portion is very small compared with the irradiation field. When the beam has a very small cross section, on the other hand, the edge of the beam swells to a degree that can no longer be neglected with respect to the irradiation field. Even when the incident energy is the same, therefore, the calibration function varies depending upon the irradiation field, and the calibration operation must be carried out for each case.